Because some of these notes are confidential. To give an example, suppose you know somebody has a rare disease. Or you know if they think bad about their boss. In case you leave your phone unlocked on the table, you don't want people to see this. The notes field is just a bit too accessible for those cases.

Re: photos, that's a good idea. I haven't felt the need myself to add photos, neither did any of the beta testers. It would be trivial to add, but any feature added is very hard to remove. I'll add it when it becomes painful not to have it ;)